Synthesis of superconducting compounds by explosive compaction of powders

ABSTRACT

SUPERCONDUCTING INTERMETALLIC COMPOUNDS, SUCH AS NB3SN ARE PREPARED BY DISPOSING A FINELY DIVIDED, STOICHIOMETRIC MIXTURE OF ELEMENTAL METAL POWDERS IN A CONTAINER, SURROUNDING THE CONTAINER WITH AN EXPLOSIVE AND DETONATING THE EXPLOSIVE. THE RESULTING EXPLOSIVE SHOCK WAVE PROVIDES THE NECESSARY REACTION CONDITIONS FOR COMPOUND FORMATION.

' Aug. 14, 1973 u. ROY ETAL SYNTHESIS OF SUPERCONDUCTING COMPQUNDS BYEXPLOSIVE commoner: 0F POWDERS Filed July 8. 1971 UPENDRA ROYTEMPERATURE K FIG.

0 OOO GUENTHER OTTO ORVIL. Y. REECE IN VE N TORS United States Patent OSYNTHESIS F SUPERCONDUCTIN G COM- POUNDS BY EXPLOSIVE COMPACTION OFPOWDERS' Upendra Roy, Guenther H. Otto, and Orvil Y. Reece,

Huntsville, Ala., assignors to the United'States of America asrepresented by the Administrator of the National Aeronautics and SpaceAdministration Filed July 8, 1971, Ser. No. 160,860

. Int. Cl. B22f 3 /08 US. Cl. 75-'-135 7 Claims ABSTRACT DISCLOSURESuperconducting intermetallic compounds such asNb Sn are prepared bydisposing a finely divided, stoichiometric mixture of elemental metalpowders in a container, surrounding the container'with an explosive anddetonating the explosive. The resulting explosive shock wave providesthe necessary reaction conditions for compound formation. I I

ORIGIN OF THE INVENTION The invention described herein was made in partby an employee of the United States Government and may be manufacturedand used by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to preparation ofintermetallic compounds and more particularly to synthesis ofsuperconducting intermetallic compounds.

Superconductors, that is, substances which undergo a transition at verylow temperatures to a state of zero electrical resistance, have foundnumerous applications in electromagnetic devices, and many researchprograms directed to further applications are currently in progress.

One of the poblems presented in connection with the most common type ofsuperconductors, namely, intermetallic compounds such as Nb sn, Nb Al,Nb Ge and the like, is their difiiculty of synthesis. Prior art methodshave generally involved isostatic or direct pressure compaction of areactant powder mixture, followed by sintering, melting or casting. Suchmethods involve large capital outlays for high-temperature furnaces,hydraulic presses, furnace atmosphere controls and related equipment.Furthermore, in some cases the incongruent melting characteristic of thepowder mixture presents a strong obstacle to formation of the desiredcompound; for example, niobium and tin have widely dilfering meltingpoints, and tin may be largely evaporated from a mixture with niobiumbefore the temperature required for intermetallic compound formation isattained. Another disadvantage of previous methods is the tendency ofthe product to pick up impurities during high-temperature processing.

SUMMARY OF THE INVENTION In the present invention superconductingintermetallic compounds are synthesized by explosive compaction of a.stoichiometric powder mixture of the constituent metallic elements. Thepowder mixture is disposed in a container and surrounded with anexplosive and the explosive is detonated. The resulting shock wavebrings about an instantaneous reaction between the metals in the powdermixture, producing an intermetallic compound in consolidated form. Thismethod is readily carried out, using inexpensive equipment, and the needfor high-temperature furnaces is avoided. Incongruent melting mixturescan be reacted by explosive compaction without loss of the lowmeltingcomponent by evaporation.

It is therefore an object of this invention to provide a method ofpreparing such compounds without prolonged heating of a reactantmixture.

Still another object is to provide a method of reactlng anincongruent-melting metallic powder mixture.

Other objects and advantages of the invention will be apparent from thefollowing detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view of apparatusfor carrying out the method of the present invention; and

FIG. 2 is a graph showing superconductivity transition curves for Nb Snsamples prepared according to the in vention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of thedrawing, a finely divided mixture 10 of metal powders in proportionscorresponding to the stoichiometry of the desired intermetallic compoundis disposed in a tubular metal casing 11 having crimpedover end portions12. The joint formed by the crimped end portions 12 is left unsealed soas to allow escape of any gases generated during explosive compaction.The powder container 11 is surrounded by an explosive 13 such asnitroguanidine which in turn is disposed in a cylindrical container 14.A conventional detonator 15 is embedded in the explosive 13 at the topof container 14. Electrical leads 16 and 17 extend from the detonator 15to a power source and switching means (not shown) for ignition of theexplosive charge.

The reactant powder mixture is made up as required for formation of thedesired intermetallic compound. For example, in the case of Nb Sn, threemoles of niobium powder are mixed with one mole of tin powder. Othersuperconducting intermetallic compounds which can be synthesized by themethod of this invention include Nb In, NbgAl, V3811, T21 511, and Nba(A10 751eo 25)- In order to undergo a complete reaction within the veryshort period of elevated temperature and pressure provided by theexplosive shock wave, the reactant metals must be in finely dividedform, and best results are obtained by using powder having a particlesize less than 325 mesh (43 microns). Thorough mixing of the reactantmetal powders by means such as tumbling is also required. The mixedpowder is placed in a container, preferably a tubular metal casing withclosed but unsealed ends as depicted in FIG. 1, and is compacted bymeans of hand tamping or the like. High-pressure compaction is notnecessary. Although the invention is not to be understood as so limited,successful results are obtained by using a tubular container .3175 to1.27 cm. in internal diameter and metal powder charge sufficient to fillthe container.

The explosive 13, which is disposed around the powder container, isselected to provide the desired shock wave characteristics, inparticular a converging shock front and a high energy density in thecenter. Explosives which can be used include nitroguanidine, dynamiteand ammonium nitrate. Nitroguanidine provides advantages in its ease ofhandling and in that its packing density can be adjusted to allowvariations in shock-wave velocity and explosive density without recourseto other explosives.

The invention is further illustrated by the following example.

EXAMPLE Fine grain (325 mesh) powders of niobium (99.99%) and tin(99.99%) were mixed in stoichiometric 3Nbz1Sn proportion and placed in acopper tube 6.2 mm. in diameter and having a 0.8 mm. thick wall. Thetube ends were pinched and crimped, and the tube was inserted centrallyinto a cylindrical container packed with nitroguanidme explosive asshown in FIG. 1 of the drawings.

The explosive charge was then detonated by means of an electricaldetonator and a tetryl booster cap positioned on top of the container.Several samples were prepared using explosive densities which variedfrom 0.2 to 0.65 gram per cubic centimeters. The product in consolidatedrod form was examined by X-ray diffraction and photomicrographs weretaken. At the lower explosive densities only a good compaction of thepowders was obtained, but at densities of 0.3 gram per cubic centimeterand higher, a distinct zone of compound formation was observed in thecenter of the same, the compound shown by X-ray diffraction to be Nb Sn.Transitions to superconductivity were measured on copper jacketedsamples using resistive and A.-C. inductance methodsTypical curves areshown in FIG. 2 of the drawing. The maximum transition temperature, 163K. (half-point of the transition curve) was obtained at an explosivedensity of 0.54 gram per cubic centimeter.

It is to be understood that the above example is merely illustrative andthat various changes and modifications in the procedures and apparatusdescribed above can be employcd by one skilled in the art withoutdeparting from the invention. It is also to be understood that theprinciples of the invention can be applied to the synthesis ofrefractory compounds other than superconducting intermetallics. Examplesof such other compounds include TaC, VC, and NbC.

What is claimed is:

1. The method of preparing a superconducting intermetallic compoundwhich comprises thoroughly mixing finely divided elemental metalconstituents of said compound at a predetermined molar ratiocorresponding to the molar ratio of metal constituents of said compound,

'4 disposing the resulting mixture in a container, surrounding thecontainer with an explosive charge sufiicient to produce upon detonationeffective temperature and pressure conditions for compound formation anddetonating said charge whereby a compound-forming chemical reactionbetween said elemental constituents is effected.

2. The method of claim 1 wherein said superconducting intermetalliccompound is a member of the group consisting of Nb Sn, Nb In, Nb Al, VSn, Ta Sn, V Al, Nb Si and s o.'15 0.25)-

3. The method of claim 2 wherein said superconducting intermetalliccompound is Nb Sn.

4. The method of claim 3 wherein said explosive is nitroguanidine.

5. The method of claim 4 wherein the density of said charge is 0.3 to0.65 gram per cubic centimeter.

6. The method of claim 5 wherein the density of said charge isapproximately 0.54 gram per cubic centimeter.

7. The method of claim 1 wherein the particle size of said elementalmetal constituents is less than -325 mesh.

US. Cl. X.R.

-l34 V, 174; 29-4205, 421 E, 599; 26484

